I am pouring a concrete floor in my basement. My questions are, do I have to put plastic over the dirt floor first, how thick should I pour the concrete and do I pour it right up against the cinder block walls?
There is an article here that addresses most of your questions.
If you like Bob Villa then check out a multipart video series here that shows the prep and pouring of a concrete floor in an existing basement. Of course they are sparing no expense in this particular project (concrete pump truck, professional install, etc.)
As far as pouring right up to the cinder-blocks, that should be fine.